Modern medicine uses X-rays more and more widely for diagnosing and treatment. In a modern medical imaging system, an X-ray tube is typically used to generate X-rays with energy lower than 500 keV (herein, energy refers to the energy of an electron beam before it hits a target), and a low-energy electron linear accelerator is used to generate X-rays with energy higher than 2 MeV. However, X-ray sources for X-rays with energy falling within a range from 0.5 MeV to 2 MeV are less common (there is a kind of X-ray tube for X-rays with an energy of 600 KeV, but such a device can be very expensive). The reason is that in this energy range, the X-ray tube is exploited to the limit, resulting in a high cost of production as the energy of the X-rays increases. An electron linear accelerator does not provide a practical solution as it is relatively expensive compared to an X-ray tube, and because an accelerator usually can only provide X-rays of a single energy. Yet X-rays with an energy falling within the range from 0.5 MeV to 2 MeV play an important role in medical imaging.
The Z value (average atomic number) of a target of medical imaging (e.g., an organism) is usually about 10. In such case, in order to ensure good imaging quality, the Compton scattering that occurs when photons interact with the target needs to be limited. The Compton scattering effect dominates when the incident photons have high energy, which will result in degradation of imaging quality. Therefore, it is considered that X-rays with an energy of about 0.6 MeV, which falls just within the foregoing range, can obtain comparatively superior imaging quality. Furthermore, imaging quality varies depending on the Z value of the target of the medical imaging. Medical imaging therefore typically requires X-rays with an energy falling within the range from 0.5 MeV to 2 MeV.
An accelerator with continuously adjustable energy can be used as an alternative to X-ray tubes, which generally do not work for the desired medical imaging energy range. There are currently several approaches for continuously adjusting energy of the accelerator. One way is to change the power fed from a power source to change the accelerating gradient of the accelerator so as to change the energy gain. This approach has a disadvantage in that the change during the low-energy phase of the gradient of the accelerating tube increases energy dispersion, and thus degrades the quality of beams. In order to address the problem of a large energy dispersion, U.S. Pat. Nos. 2,920,228 and 3,070,726 disclose an accelerator that uses two traveling wave tubes to accelerate electrons. The first traveling wave tube accelerates electrons to near the speed of light, and the second adjusts the energy by changing the RF (Radio Frequency) phase. The approach, however, has a disadvantage in that the acceleration efficiency is low due to a traveling wave accelerating structure. In order to address the problem of low efficiency, U.S. Pat. No. 4,118,653 proposes an accelerating structure by combining traveling waves and standing waves. The approach, however, has a disadvantage in that two kinds of acceleration structures are used, which results in a decentralized structure and complex peripheral circuitries. In order to have a compact acceleration structure, U.S. Pat. No. 4,024,426 proposes a standing wave accelerator using two interlaced, side-coupled substructures which adjusts the energy by changing a microwave phase difference between accelerating tubes. The approach has a disadvantage in that the accelerating tube has a complex structure that is difficult to manufacture, rendering the approach difficult to implement. In order to achieve a simple acceleration structure and a high accelerating efficiency, U.S. Pat. Nos. 4,286,192 and 4,382,208 propose an accelerator, which adds several (one or two, respectively) perturbation sticks on a coupling cavity of a side-coupled linear accelerator, the perturbation stick adjusting the phase by adjusting its insertion depth. The approach has a disadvantage in that the range for adjusting the energy is small and requires an expert to adjust the perturbation stick. In view of the foregoing disadvantages, Chinese Patent No. CN202019491U discloses a side-coupled standing wave accelerator that adjusts the energy by adjusting the accelerating gradient of two segments of accelerating tubes, respectively. But this approach too has a disadvantage in that the accelerator has a large width, the microware feeding system is complex, and it cannot provide electron beams of low energy (˜1 MeV).
In view of the foregoing, existing X-ray tubes and linear accelerators cannot cover the energy range from 0.5 MeV to 2 MeV, or have a complicated structure and are difficult to implement. Therefore, there is a need for an accelerating apparatus that outputs beams that cover the desired energy range, has a simple structure, and is easy to implement with a tolerable cost.